flash-kernel postrm.d script fails on kernel removal

[Robert C Jennings]

Context: livecd-rootfs build for ARM64 ubuntu images using the linux-generic kernel

What changed: linux-image-generic-extra-* came into being and now linux-image-generic-* depends on it. When removed the zz-flash-kernel hook for the kernel runs twice (postinst.d and postrm.d)

What happened:
 - Creating a container image (LXD, ...) livecd-rootfs starts with a server image with a kernel and removes the kernel.
 - The kernel postinst.d hook for flash-kernel (zz-flash-kernel) runs twice now (first for postinst and then for postrm). On the second call there is no kernel installed and flash-kernel does not handle being called with no kernel present; the kernel removal operation fails at this point.

Looking for a solution where zz-flash-kernel/flash-kernel is tolerant of this behavior such that kernel removal is possible.

[Loïc Minier (lool)]

The presence of a flash-kernel deb on a system is meant to imply that kernel installations/removal/upgrades should update the bootloader state. When using image building tools, flash-kernel's behavior needs to be entirely disabled, much like daemons shouldn't be started in such a chroot. To disable flash-kernel's behavior, set FLASH_KERNEL_SKIP=1 in the environment.

Another approach used commonly by image building tools is to dpkg-divert binaries (e.g. start-stop-daemon) to avoid services from being started; a similar approach could be taken to disable flash-kernel.

[Robert C Jennings (rcj)]

Loïc, thank you. We will use FLASH_KERNEL_SKIP in the build.